12. It results from very various experimental investigations some of which are mentioned above that, with a very doubtful but unique exception in the case of Fizeau's experiments on piles of glass plates, the most varied optical phenomena, whether of ray paths or of refraction, dispersion, interference, diffraction, rotation of plane of polarization, have no relation to the direction of the Earth's motion through space, though for many of them the test has been made with great precision. The most obvious conclusion from this consensus of evidence taken by itself would be the view that the Earth's motion carries the aether completely along with it, and that all the relative optical and other phenomena are therefore just the same as they would be with both the Earth and the aether at rest. Such a view is also very temptingly suggested by the absolutely negative result, up to the second order, of the Earth's motion on the Michelson interference experiment. If then we could assume that the Earth's motion produces flow, differentially irrotational according to Sir George Stokes' criterion, in the surrounding aether, but such that in all regions near the Earth's surface, up to the greatest distance at which we can explore, the aether is practically carried along bodily with the Earth, the requirements both of astronomical aberration and of the mass of negative optical results would be fully satisfied. But here we are met by various difficulties. If we assume that the aether around the Earth near its surface is carried on by the Earth as that body traverses its orbit, and also assume that at a great distance the aether is at rest, these states of motion cannot be connected without discontinuity by any possible irrotational motion of the intervening aether. The irrotational motion set up by the motion of the Earth and the surrounding shell of aether, supposed attached to it, is the same as would be set up by a moving solid in ideal frictionless liquid: the continuity of normal flow can be preserved, but there must be tangential discontinuity (slip) either at the boundary of the solid or somewhere else: this is the case whether incompressibility of the aether is assumed or not, the two sets of conditions continuity of normal flow and continuity of tangential flow being more than can be simultaneously
